The pumping services sector within the oil and gas industry injects fluid into wells to stimulate production or to encase well bore tubulars. The fluids that are pumped usually include various chemicals and solid particulates. The chemicals are added to enhance the properties of the fluids or to make them more compatible with the hydrocarbon bearing formation. The particulates that are added to the fluids are used as propping agents, diverting agents, or as extenders that reduce volumetric cost, change volumetric density, or even enhance properties of the base fluid.
Sands (silicon, ceramic, resin), glass beads, and salts are examples of particulates that are added to fracture fluids, acids, and cements. All of these products come in defined densities and size ranges. The operations that employ these materials are pre-engineered for varying concentrations during the treatment dependent on the desired final results.
Within the industry, it is desirable to monitor the quality of the fluid within the system. This includes monitoring the concentration of particulates within the fluid. Current methods for quality control of the addition of particulates includes: batch weighing, both pre and post job, mechanical metering during the addition of the particulates, or radioactive density measurements of the fluid slurries during operations.
Batch weighing provides quality control of the cumulative total product used, but does not provide quality control during on the fly operations for pre-engineered programs that vary the rate at which particulates are added during different phases of the injection.
Mechanical metering involves measuring the rate at which the particulate is added and the rate of the fluid prior to addition (clean rate) and then using these rates to calculate the particulate concentration in the slurry. The calculation for concentration is based on the knowledge of the density of both the fluid and particulate. However, mechanical metering is prone to slippage and inaccuracies due to the efficiencies of the mechanical system being employed. The quality of the measurement is therefore limited.
Another method of measuring concentration is the use of radioactive densitometers. The densitometer measures the absolute density of the slurry flowing in the pipe, and then from knowledge of the fluid density and the particulate density, the particulate concentration can be calculated.
Radioactive density measurements are the most accurate method of concentration measurements. The densities of the fluids and particulates are known prior to pumping and the radioactive density meter reads the absolute density of the slurry from which the particulate concentration can be calculated. The problem with radioactive density meters is the relative cost, management of the radioactive source, and the limitations of the meter. The limitations of the radioactive meter are its accuracy at low densities and its sensitivity when the differential density of the carrying fluid and the particulate is small.
An alternative solution taught in U.S. Pat. No. 5,390,547 to Liu is a method of splitting the phases in the fluid apart in order to calculate concentration. In Liu, the phases are split into gas/fluid or gas/free water/oil-water emulsions and the rates are individually measured. However, the solution in Liu is not practical for the measurement of particulate concentration due to the high pressures seen during injection operations.
Other solutions include a system of multiple acoustic sensors tied together via fiber optics as described in U.S. Pat. No. 6,354,147 to Gysling et al. However, the use of multiple sensors prohibitive and the system taught by Gysling is difficult to operate in the extreme mobile environment of oil field pumping operations.
A further solution includes a system that uses a transmit and receive process. U.S. Pat. No. 6,381,549 to Smith teaches a system in which a wave is transmitted, and the “echo” and transmit time is used to determine the rate and density. This system will again however be subject to high costs due to the need for multiple sensors (both transmit and receive sensors) and will again be negatively affected by the harsh mobile environment.
Other systems, including the systems taught in U.S. Pat. No. 5,741,980 to Hill et al., are also complex, making them cost ineffective and highly vulnerable to the harsh operational environment of the field.